1. Technical Field
This invention generally relates to field effect transistors, and more specifically relates to body contact field effect transistors formed in silicon-on-insulator technology.
2. Background Art
A conventional transistor has a source region and a drain region spaced apart by an intervening body region. All of these regions are planar, and are controlled by a gate. The body region is the area from which electron hole pair generation takes place that allows current to be carried between the source and drain regions underneath the gate. By contacting the body region, a charge may be applied that changes the voltage at which the transistor turns on. This is often referred to as a Vt adjustment because the threshold voltage of the device is being adjusted with this technique.
Silicon-on-insulator (SOI) technology employs a layer of semiconductor material overlying an insulation layer on a supporting bulk wafer. Typically, the structure comprises a film of crystalline silicon on a buried layer of silicon oxide on a crystalline silicon substrate. SOI technology makes possible certain performance advantages, such as a reduction in parasitic capacitance, useful in the semiconductor industry.
In a non-SOI transistor the body is automatically contacted simply because it forms part of the same silicon substrate on which all devices sit, and is either grounded via contact to the backside of the chip so the bodies of all the devices are grounded, or tied to the power supply via the N-well. On a SOI wafer, however, the body of the transistor is separated from whatever devices may be separately connected to the wafer by the buried oxide layer. SOI technology where the body is not connected to anything-called a floating body device-may suffer from the problem of hysteresis: the body retains charge and some of the electrical properties from the last time the transistor was used, interfering with subsequent use of the device.
The use of a body contact in SOI addresses this problem, and also presents other opportunities. For example, body contacts allow the threshold voltage to be changed so that standby power can be reduced for low-power applications. Body contacts in SOI have conventionally been made by creating a T-shaped structure on the diffusion, thereby creating three distinct regions: a source, a drain, and a body contact region. This approach leads to decreased performance in that it yields a greatly increased gate capacitance over a conventional device, often leading to very poor performance. Therefore, there exists a need for a body contact in SOI processes that allows precise control of the body potential but that does not lead to the poor performance that comes from high gate capacitance.
The present invention provides a body contact structure that overcomes the disadvantages of the prior art by utilizing an insulating structure between the body contact portion of the active area and the transistor portion of the active area. In particular, the present invention provides an insulative structure formed across the active area that isolates the portion of the area where transistors are formed from the portions of the active area where the body contact is formed. The body contact produced by these methods adds no significant gate capacitance to the device.
The present invention can be implemented using a variety of fabrication methods. Each fabrication method forms the insulative structure between the transistor portion of the active area and the body contact portion of the active area, but does so in a variety of different ways. One method substitutes an insulator for at least a portion of the gate layer in the regions between the transistor and the body contact. Another method removes a portion of the gate layer and replaces it with an insulative layer in regions between the transistor and the body contact. Still another forms the insulative structure by forming multiple layers, or thicker layers, of gate dielectric between the gate and the body in regions between the transistor and the body contact.